1. Field of the Invention
This invention relates, in general, to electrical inductive apparatus and, more specifically, to electrical inductive apparatus having rectangular shaped magnetic cores.
2. Description of the Prior Art
It is common in electrical inductive apparatus, such as transformers, reactors and the like, to construct a magnetic core having rectangularly shaped leg members with electrical windings, comprised of a plurality of turns of strap or sheet conductors, disposed therearound. A winding tube is disposed between the windings and the core to electrically insulate the windings from the grounded core and further serves to hold the windings in the desired rectangular shape during their sub-assembly on various coil winding machines. Typically, the winding tube is constructed of pressboard due to its superior electrical insulating characteristics.
The rectangular core type construction has been limited in the past to certain ratings since larger units have been unable to meet the short circuit withstand requirements. During a short circuit, the low voltage and high voltage windings tend to separate with the low voltage winding being compressed against the core and the high voltage winding being subject to an outward tensile force. Unless the windings are tightly wound around the core, gaps will exist between the turns of the windings which allow the windings to vibrate during a short circuit. This repeated vibration eventually breaks the insulation or the conductor thereby destroying the transformer. One of the primary reasons for the existence of voids between the turns of the windings is the dimensional instability of the pressboard material commonly used in the winding tube. Pressboard has a tendency to shrink during the curing of the transformer since the moisture absorbed by the pressboard during storage and handling is driven out by the high temperatures used in the assembly of the transformer. Further, the pressboard compresses during the processing of the transformer which opens up additional gaps or spaces between the turns of the windings. The dimensions of the pressboard winding tube vary across its length due to inherent manufacturing variations and the fact that it is difficult to bend the pressboard into a rectangular shape that is square or parallel with the edges of the core.
During the assembly of the transformer, pressure is applied to the longer axis or length of the windings to compress the windings and thereby reduce the space factor of the transformer. This eliminates the gaps between the turns of the windings along this axis of the transformer. However, the excess material that was originally present along the length of the coils appears at the ends of the windings, adjacent the width thereof, which adds additional gaps or looseness to the windings along this dimension of the transformer.
During the sub-assembly of the windings, the winding tube is initially secured around the mandrel of a coil winding machine. Due to the dimensional instability of the pressboard winding tube, in particular its varying thickness across its length and the difficulty involved in bending the pressboard into a rectangular form with parallel sides, the outer surface of the winding tube is generally not parallel with the surfaces of the mandrel or of the sheets of insulation and sheet conductor. This misalignment causes creases or wrinkles to form in the insulation and sheet conductor as they are wound around the winding tube which adversely affects the electrical characteristics of the windings. It is known that a crease or wrinkle in a sheet of paper insulation will greatly reduce its dielectric strength. Further, a crease or wrinkle could break the sheet conductor, which is typically aluminum in thicknesses of 0.005 inches to 0.040 inches. In order to reduce the occurrence of wrinkles or creases, the tension applied to the sheets of insulation and conductor during their winding around the winding tube must be reduced which, accordingly, results in a loose winding having gaps therein.
Therefore, it is desirable to provide a transformer having rectangular core legs which has a higher rating than previously attainable due to improved short circuit withstand capability. It would also be desirable to provide such a transformer that has a winding tube that is dimensionally stable over the range of conditions encountered during the construction of the transformer. It would also be desirable to provide a winding tube which has an outer surface that is parallel to the surface of the mandrel of the coil wnding machine and the sheets of insulation and conductor.